1. Field of the Invention
The present invention relates to a novel routing method specifically adapted for use with ad-hoc mobile networks and more particularly though not exclusively to a routing method where communications between source and destination mobile hosts is carried out across a conference size packet radio network of mobile hosts.
2. Status of the Prior Art
Ad-hoc mobile networks have recently become important in the field of mobile communications particularly with respect to mobile computer supported collaborative work. An Ad-hoc mobile network comprises a plurality of mobile hosts each being able to communicate with its neighbouring mobile hosts which are a single radio hop away. In such a network, each mobile host acts as a router forwarding packets of information from one mobile host to another.
Ad-hoc mobile networks differ from infra-structured wireless local area networks in that they do not have any access to base stations. Rather, in an ad-hoc mobile network, the mobile hosts have always to be able to communicate with each other over a wireless media without any infra-structured (wired) network component support. Accordingly, one of the most important features of any routing method or protocol for an ad-hoc mobile network, is the ability to adapt well to link changes, namely changes in the interconnectivity between mobile hosts due to mobile hosts' migrations. Ideally, mobile hosts should not spend most of their time updating and computing routes in sympathy with other mobile hosts' movements. However, conventional distributed routing schemes attempt to maintain consistent routing information by performing periodic link and topology updates. These updates are undesirable because of the frequent link changes occurring in ad-hoc mobile networks, which can result in an enormous number of transmissions over the wireless media to propagate and update routes. This is highly impractical, very inefficient and results in low data throughput in an environment where radio bandwidth and battery power are scarce resources.
One of the earliest deployments of a regional-wide wireless data network was the ARPANET Packet Radio Network (PRN) by Kohn, Gronemeyer, Burchfiel and Kunzelman. As shown in FIG. 1, all components (repeaters R. terminals T and stations S) in a PRN can be mobile or certain components can remain fixed while others are moving. There are two approaches used in a PRN for routing and packet forwarding. In "point-to-point" routing, the station computes all the routing information and the decision is either distributed to the repeaters involved in the route or to the source. This scheme is only suitable for slow moving user terminals. However, in "broadcast routing", each packet radiates away from the source with a wave-front-like propagation. Since no station needs to be present to compute routes, the destination address serves to identify the intended recipient. For fast moving user terminals, broadcast routing is preferred over point-to-point routing as it avoids the need to process rapidly changing routes.
In the connectionless approach to packet forwarding, some background operation is required to maintain up-to-date network topology and link information in each node. Accordingly, as network topology changes, the background routing traffic required in using the connectionless approach can be substantial. The connectionless approach is commonly associated with broadcast routing, where each packet carries sufficient routing information for it to arrive at the destination. However, in the connection-oriented approach, an explicit route establishment phase is required before data traffic can be transported. The connection-oriented approach is commonly associated with point-to-point routing, where each node in a route has a look up table for forwarding incoming packets to the respective out-going links. The disadvantage of the connection-oriented approach is that if the network topology changes, a route re-establishment phase is required.
Several ad-hoc mobile routing schemes have evolved over the past few years. Most of these schemes are based on either broadcast or point-to-point routing using either the connectionless or connection-oriented packed forwarding approach.
The "Layer Net" self-organising protocol proposed by Bhatnagar and Robertazzi uses a connectionless packet forwarding approach. Broadcast routing is used for the initial network connectivity construction and the subsequent topology maintenance as a result of nodes' movements and link changes. Network topology updates have to be performed in sympathy with link changes and routes are not constructed based on demand. Accordingly, the overall signalling traffic can be quite substantial.
Cluster-based routing by Krishna, Chatterjee, Vaidya and Pradhan uses the broadcast routing and connectionless packet forwarding approach. Cluster-based routing relies on existing routing schemes such as link-state or distance-vector routing to derive network topology and link information. In addition, a clustering methodology is used to reduce the number of updates due to mobile hosts' migrations. Routes are constructed between all pairs of nodes and route maintenance is essentially cluster maintenance. The disadvantage of cluster-based routing is that the method is inefficient.
Source-initiated distributed routing by Corson and Ephremides uses a combination of point-to-point and broadcast routing using the connection-oriented packet forwarding approach. Here routes are initiated by the source and are constructed based on demand, and so this method forgoes the need to constantly propagate up-to-date routing information throughout the network. However, because alternate route information is used during route re-construction, problems associated with stale routes exist.
The Destination Sequence Distance-Vector routing scheme proposed by Perkins and Bhagwat is an enhancement of the existing distance-vector Bellman-Ford routing, so that ad-hoc mobile networking can be supported. Because each mobile host has to periodically advertise its view of the network topology, this scheme is inefficient. Similar to cluster-based routing, the broadcast routing and connectionless packet forwarding approach is adopted.
Dynamic source routing for mobile networks by Johnson avoids periodic route advertisements because route caches are used to store source routes that a mobile host has learnt over time. A combination of point-to-point and broadcast routing using the connection-oriented packet forwarding approach is used. Routes are source-initiated and discovered via a route discovery protocol. With source routing, the sender explicitly lists the route in each packet's header, so that the next-hop nodes are identified as the packet travels towards the destination. Cached route information is used and accurate updates of these route caches are essential, otherwise routing loops can occur. Since the sender has to be notified each time a route is truncated, the route maintenance phase does not support fast route reconstruction.